1. Field of the Invention
The present invention generally relates to a copying machine of a type employing a movable optical system and, more particularly, to a lens repositioning device used in the copying machine for repositioning a lens assembly for the selection of one of copying magnification factor.
2. Description of the Prior Art
The prior art copying machine of a type employing the movable optical system, that is, in which the optical system including an illuminator, a reflecting mirror assembly and a projecting lens assembly are supported for movement so as to scan an original document to be copied, is illustrated in FIG. 4 of the accompanying drawings.
Referring first to FIG. 4, the copying machine comprises a document support 31 for supporting an original document 39 to be copied, which document 39 is scanned by an optical system. The optical system includes an illuminator lamp 32, a first reflecting mirror 33, a first reflecting mirror pair 34, a second reflecting mirror pair 36 and a second reflecting mirror 37, all of these mirrors 33 to 37 being so arranged as to direct the rays of light reflected from the original document 39 towards a photoreceptor drum 38 as will be described later. Specifically, the illuminator lamp 32 and the first mirror 33 are integrated together in a single carriage movable underneath the document support 31 for scanning the original document 39 placed on the document support 31. The first mirror pair 34 is also supported for movement in a direction parallel to the direction of movement of the carriage. The second reflecting mirror 37 is fixedly supported. Both of the lens assembly 35 and the second lens pair 36 are movably supported, however, they can be retained at a selected one of a plurality of positions during the scanning of the original document 39 while the movement of both of the lens assembly 35 and the second lens pair 36 take place only when the copying magnification at which an image of the original document 39 is desired to be copied is changed.
When the scanning of the original document 39 takes place, rays of light emitted from the illuminator lamp 32 sequentially and successively illuminate the original document 39 and are reflected thereby towards the first mirror pair 34 via the first mirror 33. The rays of light carrying an image of the original document 39 are then reflected back by the first mirror pair 34 towards the photoreceptor drum 38 through the lens assembly 35 by way of the second mirror pair 36 and then by way of the second mirror 37.
During this operation, the first mirror 33 integrated together with the illuminator 32 is driven in a scanning direction at a predetermined scanning speed whereas the first mirror pair 34 is driven in the same direction at a speed which is equal to half the predetermined scanning speed. Accordingly, the distance a of the path of travel of the rays of light from the original document 39 to the lens assembly 35 through the first reflecting mirror 33 and the first mirror pair 34 remains constant regardless of the position of the carriage relative to the original document 39. Also, the distance b of the path of travel of the rays of light from the lens assembly 35 to the photoreceptor drum 38 through the second mirror pair 36 and the second reflecting mirror 37 remains constant because, during the scanning operation, both the lens assembly 35 and the second mirror pair 36 are fixed. Thus, the distances a and b of the respective paths of travel of the rays of light have a predetermined relationship with respect to each other during the scanning of the original document 39.
It is to be noted that, in the copying machine of a type having a movable document support, the optical system does not move and, therefore, the distances a and b of the respective paths of travel of the rays of light are fixed to an equal length.
The reason why the distances a and b must have a predetermined relationship will now be discussed. Assuming that the image of the original document 39 is desired to be projected onto the photoreceptor 38 through the lens assembly 35, and assuming that the lens assembly 35 has a focal length f, the following relationship must be satisfied. EQU 1/a+1/b=1/f (1)
If the focal length f of the lens assembly 35 is assumed to be fixed, the copying magnification m at which the image of the original document is projected by the lens assembly onto the photoreceptor drum 38 can be expressed as follows. EQU m=b/a (2)
Accordingly, in order for the equation (1) above to satisfied while the copying magnification m is retained at a constant value, the distances a and b must be held constant at respective values.
A method employed in this type of copying machine for changing the copying magnification m will now be described.
If the copying magnification m is x1, the following relationship establishes between the distances a and b in view of the equations (1) and (2) above. EQU a=b=2f (3)
Alternatively, if the copying magnification m changes, the distances a and b must be modified so as to satisfy the following relationship. EQU a=f(1+1/m) (4-1) EQU b=f(m+1) (4-2)
Thus, once the copying magnification m is chosen, the lens assembly 35 has to be moved so that the distance a of the optical path along which the rays of light travel from the original document 39 to the lens assembly 35 can satisfy the equation (4-1) and, at the same time, the second mirror pair 36 has to be moved so that the distance b of the optical path along which the rays of light travel from the lens assembly 35 to the photoreceptor drum 38 can satisfy the equation (4-2). It is to be noted that, where instead of the use of a combination of the second mirror pair 36 and the second reflecting mirror 37 a single reflecting mirror is employed for projecting the rays of light direct onto the photoreceptor drum 38, not only must the single reflecting mirror be moved, but the angle of inclination of the single reflecting mirror must also be adjusted.
Apart from the above, when it comes to the manner in which both an original document to be copied and a copying paper are set in the machine, a center-alignment method and a side-alignment method are well known. According to the center-alignment method, the original document 39 is required to be placed on the document support 31 with the center line of the original document 39 aligned with the center line of the document support 31 which extends along the scanning direction while the copying paper 40 is required to be set in the machine so that the center line of the copying paper 40 can align with the center line of the photoreceptor drum 48 intermediate the length of the photoreceptor drum 48. In the copying machine of the center-alignment system, even when the copying magnification m is changed, the optical center line of the image is aligned with the optical center line of the optical system at all times regardless of the size of any one of the original document 39 and the copying paper 40 and, therefore, it suffices for the lens assembly 35 to be moved straight along the optical axis of the optical system.
It is to be noted that the term "optical center line of the optical system" referred to above and hereinafter used is intended to means the imaginary line drawn on the optical path so as to extend between the center line of the document support 31 extending along the scanning direction and the center line of the photoreceptor drum 38 intermediate the length of such drum 38. It is also to noted that the term "optical center line of the image" also referred to above and hereinafter used is intended to means the imaginary line drawn on the optical path so as to extend between the center line of the document 39 extending along the scanning direction and the point of exposure on the photoreceptor drum which corresponds to the center line along a direction of conveyance of the copying paper 40.
In the center-alignment system, the optical axis of the lens assembly 35 must be aligned with the optical center line of the image at all times. However, it has been found that, according to the center-alignment system, and when the original document 39 is to be placed on the document support 31, an operator of the copying machine is required to place the original document 39 on the document support 31 with reference to scale calibrations so that the center line of the document 39 can be aligned with the center line of the document support 31. Thus, the center-alignment system has a problem in that the operator may endeavor to carefully place the original document on the document support.
In view of the foregoing, an increasing number of recent versions of the copying machine employ the side alignment system. The side-alignment system is such that, one of the opposite side edges of the document support 31 which extend along the scanning direction is used as a reference side edge against which a corresponding side edge of the original document 39 is abutted during the placement of the document 39 on the document support 31 and, in a similar manner, the copying paper 40 is set in the machine so that the copying paper 40 can be conveyed over the photoreceptor drum 38 with one side edge thereof aligned with one end portion of the photoreceptor drum 38, that corresponds to the reference side edge of the document support 31, regardless of the size of the copying paper.
In the copying machine employing the side-alignment system, since the placement of the original document on the document support can be accomplished merely by aligning one side edge of the original document against the reference side edge of the document support 39, the operator of the machine will not be burdened so much as with the copying machine employing the center-alignment system.
In the copying machine of the type employing the side-alignment system, when the copying magnification m is changed, the optical center line of the image will be no longer aligned with the optical center line of the optical system, and vice versa. Because of this, the movement of the lens assembly 35 straight along the optical center line of the optical system results in a deviation of the optical center line of the image from the optical axis of the lens assembly 35 and an electrostatic latent image corresponding to the image of the original document will be formed on the photoreceptor drum 38 in a biased fashion, that is, biased in the lengthwise direction of the photoreceptor. Consequently, the resultant copy will show the image reproduced on the copying paper 40 in the correspondingly biased fashion.
In view of the foregoing, in the copying machine of the type employing the side-alignment system, the lens assembly 35 when moved along the optical center line of the optical system has to be also moved in one plane in a direction perpendicular to the optical center line of the optical system so that the optical axis can be aligned with the optical center line of the image, as discussed in the Japanese Patent Publication No. 54-1178 published in 1979.
The manner in which the lens assembly 35 is moved when the copying magnification m is changed in the copying machine employing the side-alignment system will be discussed in detail with reference to FIG. 5. It is to be noted that, in FIG. 5, for the purpose of discussion, the optical center line of the optical system is depicted a extending in a plane and the copying paper 40 is assumed as being placed on the point of exposure on the photoreceptor drum 38.
It is assumed that, when the copying magnification m is chosen to be xl (equal size reproduction), the placement of the original document 31 with one side edge aligned with the reference side edge of the document support 39 and the subsequent positioning of the lens assembly 35 at a lens position Pa on the optical center line of the optical system would result in the reproduction of an equal size of the image on the copying paper 40a having one side edge aligned with one end of the photoreceptor drum 38. In such a case, the optical center line of the image and the optical center line of the optical system ar aligned with each other and are also aligned with the optical axis of the lens assembly 35 as clearly shown in FIG. 5. However, when the copying magnification is desired to be changed to .times.0.8 (80% reproduction), the lens assembly 35 has to be moved a distance equal to 0.25f in a direction, shown by the arrow X in FIG. 5, along the optical center line of the optical system and close to the copying paper 40 to satisfy the equation (4) above. In such a case, since the copying paper 40b will be of a size 0.8 times the size of the copying paper 40a, the alignment of one side edge of the copying paper with the reference side edge of the document support 39 will result in a deviation of the center line of the copying paper from the optical center line of the optical system. Accordingly, the lens assembly 35 is required to be moved not only in a direction close to the copying paper, but also in a direction, shown by the arrow Y, perpendicular to the optical center line of the optical system. A symbol Pb shown in FIG. 5 represents the position assumed by the lens assembly 35 when the latter is so moved. Similarly, when the copying magnifications m are chosen to be .times.0.6 and .times.0.5, respectively, the lens assembly 35 is moved not only in the direction shown by the arrow X, but also in the direction shown by the arrow Y to respective lens positions indicated by Pc and Pd.
The distance between the original document 39 placed on the document support 31 and the copying paper 40 takes a minimum value when the distance a of the optical path and the distance b of the optical path shown in the equation (3) above coincide with each other. Because of this, the change of the copying magnification m results in the positioning of the copying paper 40b, 40c or 40d at a location spaced from the position of the copying paper 40a in the direction shown by the arrow X. The adjustment of the distance b of the optical path is accomplished by moving the second mirror pair 36 so as to satisfy the equation (4) above.
As clearly shown in FIG. 5, the lens positions Pa, Pb, Pc and Pd lie on a straight line. Because of this, assuming that the straight line passing through respective points closest to the lens positions Pa, Pb, Pc and Pd, when determined by the use of, for example, a method of least squares, extends in a direction shown by the arrow X' and inclined at an angle .theta. relative to the direction shown by the arrow X, even the movement of the lens assembly 35 in the direction shown by the arrow X', that is, along the straight line so determined, is not satisfactory and does not bring the lens assembly to the associated lens position Pa, Pc, Pc or Pd unless the lens assembly 35 is moved a slight distance in the direction shown by the arrow Y.
It is to be noted that, in FIG. 5, the original document 39 shown therein is assumed to be of a maximum size which the document support 31 can accommodate. Therefore, as far as the original document 39 of the maximum size is concerned, the alignment of one side edge of the original document with the reference side edge of the document support 31 results in the alignment of the center thereof with the optical center line of the optical system. However, where the original document 39 actually placed on the document support 31 is of a smaller size, the foregoing explanation can be equally applicable if one side of the original document is abutted against the reference side edge of the document support 31 during the placement of the original document on the document support.
From the foregoing reasoning, a lens repositioning system hitherto used in the copying machine of the type wherein the original document is laid on the document support with its one side held in abutting relation to the reference side edge of the document support 31 is so designed that the lens assembly 25 can be driven in a plane not only in the direction shown by the arrow X' and inclined relative to the optical center line of the optical system, but also in the direction perpendicular to the optical center line of the optical system.
Where the lens assembly 35 used in the machine is of a type wherein the focal length f is variable, that is, a zoom lens assembly, the position of the copying paper 40a, 40b, 40c or 40d in the direction shown by the arrow X may remain unchanged. However, the change of the copying magnification requires a corresponding adjustment of the focal length and, at the same time, the movement of the lens assembly 35. When it comes to the movement of the lens assembly 35 where the latter is a zoom lens assembly, the lens assembly 35 is required to be moved not only in the direction of the arrow X', but also in the direction of the arrow Y in a manner as hereinbefore discussed for the same reason as hereinbefore discussed.
The prior art lens repositioning mechanism for repositioning the lens assembly 35 in the manner as hereinbefore described such as disclosed in, for example, the U.S. Pat. No. 4,552,453, issued Nov. 12, 1985, is illustrated in FIG. 6, reference to which will now be made.
As shown in FIG. 6, the lens assembly 35 is rigidly supported by a generally elongated lens holder 41 which is in turn slidably mounted on a shaft 42 extending in a direction perpendicular to the optical axis of the lens assembly 35 and in the direction shown by the arrow Y. This lens holder 41 is displaceable in a direction along the shaft 42, however, it is normally biased by a spring member 43 in the direction of the arrow Y. The lens holder 41 has one of the opposite ends provided with a roller 44. Positioned on one side of the roller 44 opposite to the lens holder 41 is a guide bar 51 extending generally in the direction of the arrow X' while inclined at the angle 8 relative to the direction of the arrow X and also relative to the optical axis of the lens assembly 35. The guide track 51 is fixedly supported by a machine framework (not shown). At least one lateral surface of the guide track 51 which is held in contact with the roller 44 is not a straight surface and constitutes a cam surface so curved and so shaped as to permit the lens assembly 35 to assume selectively any one of the lens positions Pa, Pb, Pc and Pd discussed with reference to FIG. 5.
The shaft 42 referred to above is secured at its opposite ends to a lens drive member 45 having one end slidably mounted on a shaft 46 extending in a direction parallel to the direction of the arrow X'. The shaft 46 has its opposite ends rigidly secured to the machine framework (not shown) of the copying machine. The other end of the lens drive member 45 is provided with a roller 47 rotatably resting on a guide rail 48 extending parallel to the shaft 46. A traction cable 50 has its opposite ends rigidly secured to a cable fixture 49 extending outwardly from the end of the lens drive member 45 adjacent the shaft 46. A substantially intermediate portion of said cable 50 is turned around a drive shaft of a drive motor (not shown) through a number of idle pulleys (also not shown) so that during the operation of the drive motor the lens drive member 45 can be pulled by the traction cable 50 to move along the shaft 46.
The prior art lens repositioning mechanism of the construction shown in FIG. 6 operates in the following manner.
As the traction cable 50 is driven in one direction by the drive motor (not shown) when one of the available copying magnifications is selected, the lens drive member 45 having the cable fixture 49 connected to the traction cable 50 is driven along the shaft 46 in a direction dependent on the direction of rotation of the drive motor. As the lens drive member 45 is driven, for example, in the direction of the arrow X', the lens holder 41 is moved in the direction of the arrow Y along the shaft 42 with the roller 44 constantly held in contact with the cam surface of the guide rod 51 by the action of the spring 43. Thus, the movement of the lens drive member 45 in the direction of the arrow X' results in the movement of the lens holder 41 in the direction of the arrow Y.
In this way, the prior art lens repositioning mechanism is so designed and so structured as to permit the lens assembly 35 to be moved along a curved path containing the lens positions Pa, Pb, Pc and Pd.
It has, however, been found that the prior art lens repositioning mechanism has the following problem. The lens repositioning mechanism is of the construction wherein the lens holder 41 is slidably mounted on the shaft 42 which is in turn mounted on the lens drive member 45 slidably mounted the shaft 46 Therefore unless the mounting of these elements is carefully exercised, it would be difficult for the optical axis of the lens assembly 35 to be held in parallel relationship with the optical center line of the optical system at all times regardless of the displacement of the lens assembly. Therefore, the prior art lens repositioning mechanism requires a highly precise machining procedure to manufacture the various component parts and also a complicated assembling procedure.
Also, in the prior art lens repositioning mechanism, not only is the drive of the lens assembly 35 effected solely by he traction cable 50 having its opposite ends rigidly connected to the cable fixture 49 integral or fast with the lens drive member 45, but also the persistent alignment with the optical axis of the lens assembly 35 in a predetermined direction is achieved solely by the slidable mounting of the lens drive member 45 on the shaft 46. Because of this, it has been observed that the shaft 46 tends to be excessively loaded to such an extent that, after a long period of use, the lens drive member 45 may fail to operate properly and smoothly.